OFDM systems are well known. Various techniques have been used for synchronisation of OFDM receivers. Some of these techniques require transmission of a special synchronisation signal. Other techniques rely on a standard OFDM signal, in which a complete symbol comprises a “useful part” and a “guard space”, the guard space sometimes being referred to as a guard interval, cyclic extension or cyclic prefix.
The guard space precedes the useful part of the symbol and contains a repeat of the data at the end of the useful part. (This is equivalent to having a guard space after the useful part, containing data which is the same as that at the beginning of the useful part.)
Synchronisation techniques which rely upon the duplicated data in the guard space generally operate by performing a cross correlation between complex samples spaced apart by the length of the useful part of the symbol. This generates a timing pulse which is used in Fourier Transformation of the received signal. The timing of the pulse is such that the Fourier Transform window contains only data from a single symbol.
If the timing is incorrect, inter-symbol-interference (ISI) occurs. However, the use of the guard space allows a certain amount of variation in the timing of the pulse while still avoiding ISI. The guard space should be longer than the longest expected spread of delays amongst signals received via different paths. The guard space is relatively small compared with the useful part of the signal; typically, the guard space may contain Nu/32, Nu/16, Nu/8 or Nu/4 samples, where Nu is the number of samples in the useful part of the symbol.
It would be desirable to provide a simpler and less expensive technique for synchronisation, and preferably one which can produce more accurate results.
Aspects of the present invention are set out in the accompanying claims.
According to a further aspect, a synchronisation pulse for a Fourier Transform demodulator is generated by taking absolute values of complex samples of an OFDM symbol, determining the difference between absolute values which are separated by the length of the useful part of the signal, and generating the synchronisation pulse in response to determining the time at which there is a substantial increase in the difference value.
The invention also extends to apparatus for generating such a synchronisation pulse, and to a method and an apparatus for receiving an OFDM signal using such techniques.
Using the method of the invention, the difference between the absolute values of the complex values will be very small when those complex values are equal. Thus, this difference will remain small whenever the samples in a guard space are subtracted from the corresponding values within the useful part of the symbol. When the values cease to be small, this represents the end of the symbol, and this point can be used to allow an FFT demodulator window to be aligned with the OFDM symbol.
It is found that, for example because of slight drift in the local oscillator, the complex samples of an OFDM signal can suffer different phase rotations within the period of a single symbol. Thus, although nominally the data in a guard space should correspond to part of the data in the useful part of the symbol, the actual complex values derived from the signal may differ because of this phase rotation. However, by taking the absolute values of the complex samples, such differences are reduced substantially. Furthermore, because the system handles real, rather than complex, values, data storage requirements are reduced. The complexities and inaccuracies associated with multipliers can also be avoided. The arrangement can also produce better accuracy than these systems which rely on integrating the output of a cross-correlator, and better acquisition times than prior art systems relying on phase locked loops (PLL's).
Preferably, the difference values are integrated over a number of symbols. Preferably this is achieved using an infinite impulse response (IIR) filter.